A photo sensor is a sensor that uses a photo element as a detection element. The photo sensor may initially convert measured changes into optical signal changes, and then convert the optical signal to an electric signal by the photo element. Photo sensors are widely used in heart rate detectors. A heart rate detector can detect a person's heart rate, such that people can determine their own health status based on the detected heart rate. Systole and diastole of the heart causes blood to flow in the blood vessels, and the amount of blood flowing through the blood vessels correspond to different reflectivity. A heart rate detector may calculate the current heart rate by detecting fluctuations of the reflected light.